Extreme weather events occur frequently as global warming raises the temperature of the earth. Rivers are often flooding, and the flooding becomes more and more serious in urban areas. Therefore, it is an urgent issue to monitor the water level of a river or a reservoir, or in an area which is easily flooded. In addition, water level or liquid level monitoring is required in other technical fields as well, for example, monitoring the level of a liquid or water for industrial use. The water level or liquid level measurement is very important in many fields. Therefore, it is necessary to develop a low-cost and efficient liquid level measuring device for serving the needs of these industries.